Negative STFT Both Banks - Can't Find the Reason

[dm290]

Hi all,

New to tuning and learning, as well as working on cars. Currently trying to track down an issue with my LS3 416 stroker in my '03 C5. Occasionally on throttle application my STFTs will go very negative, sometimes up to -30%, my AFR gauge will read max lean for about a second or two then it's back to normal. Sometimes it happens at idle too. Unsure if it's a tuning issue or a mechanical issue at this point.

I've been working with a tuner/shop to identify the cause but I've come here for outside input since we're both stuck.. At first we thought it was just a vacuum leak, it's not; I checked hoses, smoke tested, sprayed carb cleaner, then took the intake manifold off and checked runners, and there's nothing wrong. Throttle body seems fine. I feel like I'm just chasing ghosts and I'm tired of throwing money at it for nothing - it's been a long battle with this car and this is the last thing to fix before I can move to the transmission. /vent

Plugs, coils and wires got 100 miles on them this time around. Last plugs with about 1000 miles all looked like they were running lean (white tips).
During idle it smells lean but AFR reads ~0.8 λ

My injectors are pretty damn new, ID1050s.
FAST LSX 102mm Intake
LS3 card style MAF -- fairly new
Cam specs: 244/250 .641/.644

I can provide any other engine specs if needed or datalog anything else.. Dropped an idle & a pull log file below w/ the current tune.

Would appreciate any insight to this issue. Thank you greatly

[gametech]

Looks like you just need to finish tuning it. Once that is done, you won't be trying to trim 10-15 percent of your fuel out. Once the VE and MAF are tuned close enough to get rid of most of the fuel trims, then you can see if you have a real problem. Just be aware that a cam with that much overlap will give false o2 readings at idle, but that is a whole other issue.

[Alvin]

This would likely run a lot better in speed density for a start..

Also would likely do better in open loop. With this much over lap the stock narrow bands will think it's rich/lean when in reality it's just the cam's overlap fooling it.

[edcmat-l1]

Plugs, coils and wires got 100 miles on them this time around. Last plugs with about 1000 miles all looked like they were running lean (white tips).

They're supposed to look like that.

During idle it smells lean but AFR reads ~0.8 λ

You can't smell lean. Just like you can't smell rich.

This needs to be in speed density and that VE table is not correct by any means.

And the transient min fuel needs to be dropped. That's a big problem with this with those big injectors.

[dm290]

I'm working off the tune from my tuner when he dyno'd it with the original FAST injectors.. This is a newer issue after swapping injectors, he couldn't figure the issue out and I believe I ruled out everything mechanical so that's why I was digging into it. Still learning, so if the tune seems off, happy to tackle it

Transient fuel though - thank you!!! Slipped my mind and reading around it seems it slips a lot of people's minds too haha
I was told if it burns your eyes at idle, the car runs lean.. And here's what my plugs looked like, I can say for my last ones did not look like that (before I had this issue) and everybody was telling me these were bad.


Adjusting TF alone has appeared to fix the issue, STFTs seem normal and it isn't pulling but ~10% unless on decel after a short pull and the hesitation/stumbling is gone completely.. 1 more weird issue to solve (hard to describe without a video, just a sound on accel) and I think I'm finally done with this thing! I appreciate the input from y'all, got me one step closer to being able to enjoy this damn car for once